This invention relates generally to inkjet printing mechanisms, and in particular to techniques for measuring a pen-to-paper spacing.
Inkjet printing mechanisms such as thermal inkjet printers and piezoelectric printers use printheads to eject drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a media sheet. Each printhead is formed with very small nozzles through which the ink drops are fired.
As shown in FIG. 2, in this application, a pen-to-paper spacing (PPS) 205 is defined as the spacing between the printhead 110 of an inkjet printing mechanism and an upper surface 203 of a media sheet 201 in a print zone 106. It is understood that the large variations in part manufacturing and the dependency of print quality on PPS make it a critical parameter to be adjusted at the printer manufacturing stage.
Present day methods use expensive tools such as laser-based linear variable differential transducers (LVDTs) or high precision mechanical LVDTs to measure the PPS. This process includes multiple measurements across the print zone on a special calibrated media sheet being fed through the printing mechanism.
However, the use of these additional tools adds extra costs to the printer manufacturing process, apart from making PPS measurement a very manual and time-consuming procedure, and requires constant operator involvement. All these may not be desirable to the manufacturer.
Therefore, there is a need for a more convenient and less expensive way of measuring the PPS in an inkjet printing mechanism.
According to an aspect of the invention, a method for measuring a pen-to-paper spacing in an inkjet printing mechanism is provided. Firstly, a first nozzle of a printhead in the printing mechanism ejects a first ink drop onto a medium at a first ejection velocity when the printhead moves at a first moving velocity along a scanning axis and when the printhead reaches a first position along the scanning axis. Subsequently, a second nozzle of the printhead ejects a second ink drop onto the medium at a second ejection velocity when the printhead moves at a second moving velocity along the scanning axis and when the printhead reaches a second position along the scanning axis. The spacing between the two nozzles along the scanning axis and the spacing between the two positions along the scanning axis are predetermined. Then the spacing between the first and second ink drops on the medium along the scanning axis is measured. The pen-to-paper spacing can be determined from the first and second moving velocities, the first and second ejection velocities, the spacing between the first and second nozzles along the scanning axis, the spacing between the first and second positions along the scanning axis and the spacing between the first and second ink drops on the medium along the scanning axis.